The present invention relates to a printer equipped with a mechanism for shifting across a printing head a printing ribbon that is broader than the printing head.
This kind of a printer is known as disclosed, for example, in Japan Published Unexamined Patent Application No. 59-78882. Conventional printers of dot impact type are constructed so as to feed a printing ribbon broader than a printing head in a direction of printing lines and to shift the ribbon by a stepping motor or the like in a direction of ribbon width orthogonal to printing lines.
Such shifting will be explained with reference to FIG. 7. A printing ribbon 200 is fed in a direction shown by an arrow a, and, after one line is printed, the ribbon 200 is shifted in a direction of its width b or c by the amount of shift corresponding to a fixed number of pulses giving to a stepping motor in a shape of steps (n represents positions of steps). Oblong printing faces 201 which can be printed in case of impacting all printing wires of a printing head are shifted reciprocally within the area of 12 steps in a direction of the ribbon width. In this way, the printing ribbon 200 can be efficiently consumed. By shifting an amount of three steps, the ribbon 200 is shifted the longitudinal width of the printing face 201.
The printer described in the above, however, has the following problems.
The printing areas A, B through O are different in frequency of use by the printing head during one cycle of reciprocation of the printing face 201 on the ribbon 200. In other words, the area A or O is used once, the area B or N thrice, the area C or M five times and the areas D through L are used six times per one cycle. For this reason, the printing faces 201 strike more in the central region of the ribbon 200, while striking less in the top and bottom regions of the ribbon 200. The areas A, B, N and O where usage frequency is less than 3 times occupy 4/15 or 26.6% of the effective usage area of the ribbon in the direction of width.
Accordingly, when the ribbon 200 is used repeatedly, the central region is used more than the top or bottom region. As a result, printing effects are dark and thick in the top and bottom regions while light and thin in the central region.
In the top and bottom regions of the ribbon, which are not used frequently, the ink remaining nearby blots at the position where the ribbon is impacted by the printing wire, resulting in different shade of the ink of one character printed.